The inventive concept relates to transistors and methods of fabricating the same and, more particularly, to high electron mobility transistors and methods of fabricating the same.
A high electron mobility transistor (HEMT) is a kind of compound semiconductor device. Generally, one or more layers included in the HEMT have lattice constants very different from those of other layers, such that a structure of a material used as a channel layer may be modified. A mobility of electrons in the channel layer of the HEMT may be improved by the stress modification according the lattice mismatch, such that an operating speed of the HEMT may be improved.
It is difficult to grow a substrate for the HEMT. However, the HEMT has higher power and improved noise characteristics by an increase of the density of charges transmitted to the channel layer and by the high electron mobility. Thus, the HEMT may be operated at a high frequency. The HEMTs have excellent electron speed characteristics as compared with an electronic device using silicon, so that they are widely used in device applications of microwave and millimetric wave bands.
Gallium nitride (GaN) corresponding to a nitride semiconductor is a direct transition type semiconductor. The gallium nitride is attractive as a new material of high frequency electronic devices because of its a high electric field electron mobility (e.g., 2×10−7 cm/s) and a high breakdown electric field (e.g., 3×106 V/cm). The nitride semiconductor may be realized as a heterojunction structure such as aluminum-gallium nitride/gallium nitride (AlGaN/GaN). Additionally, the nitride semiconductor may be selective doped. Thus, the nitride semiconductor maybe designed to have an optimized structure for a high speed. Due to the above physical properties of the nitride semiconductor, an electronic device including the nitride semiconductor may improve trade off relationship between a cutoff frequency (ft) and a breakdown voltage (VBV) which is a problem in in a conventional field effect transistor (FET). Thus, the electronic device including the nitride semiconductor may have high voltage and high frequency characteristics.
If a FET includes the nitride semiconductor which has a great power density as well as the high voltage operation characteristic and is used in an amplifier module, a peripheral circuit such as a power distribution synthesis circuit and a direct current (DC) voltage converting circuit may be omitted or simplified in the amplifier module. Thus, it is possible to realize a high power amplifier module having high power usage efficiency.
Additionally, a high speed semiconductor device may use a T-shape, Y-type or mushroom-type gate having a wide cross-sectional area in order that a resistance of the gate is reduced to improve its noise characteristic. The T-type, Y-type, or mushroom-type gate is generally formed by an electron beam (E-beam) lithography method or a photolithography method.
However, a resolution of the photolithography method may be insufficient for forming a gate electrode having a fine width. Thus, the E-beam lithography method may be mainly used to form the gate electrode. Generally, a double-layered or triple-layered photoresist layer in the E-beam lithography method.